Disable socket-pair test because it is flaky

Summary: It's failing a bunch on sandcastle

Reviewed By: @alexmalyshev

Differential Revision: D1741089
Cleanup and document VariableUnserializer API

Summary: Brought VariableUnserializer up to our current coding standards,
documented the API, and in the process found some minor simplifications (an
unused method, unused member, and constructor that didn't add much value).
Split out the inline methods to improve readability.

Reviewed By: @edwinsmith

Differential Revision: D1738354

Signature: t1:1738354:1418570047:84f18bac7ef3cacac159f66fae9f9c11d83b471e
Don't print key's local id in IterData if it's invalid

Summary: It prints as 'IterInit<Bool,0::4294967295::2> t43:Arr, t0:FramePtr'
otherwise.

Reviewed By: @jdelong

Differential Revision: D1737610

Signature: t1:1737610:1418506183:325bddb2e47b5e85ff7b285b6c9c96ced5b932a0